Editing Exhaust gas recirculation (section)

== History ==
The first EGR systems were crude; some were as simple as an [[Calibrated orifice|orifice jet]] between the exhaust and intake tracts which admitted exhaust to the intake tract whenever the engine was running. Difficult starting, rough idling, reduced performance and lost fuel economy inevitably resulted.<ref name=Petersen>{{cite book | editor-last = Rosen | editor-first = Erwin M. | title = The Peterson automotive troubleshooting & repair manual | publisher = Grosset & Dunlap, Inc. | year = 1975 | isbn = 978-0-448-11946-5 }}</ref> By 1973, an EGR valve controlled by [[manifold vacuum]] opened or closed to admit exhaust to the intake tract only under certain conditions. Control systems grew more sophisticated as automakers gained experience; Volkswagen's "Coolant Controlled Exhaust Gas Recirculation" system of 1973 exemplified this evolution: a coolant temperature sensor blocked vacuum to the EGR valve until the engine reached normal [[operating temperature]].<ref name=Petersen/> This prevented driveability problems due to unnecessary exhaust induction; {{NOx}} forms under elevated temperature conditions generally not present with a cold engine. Moreover, the EGR valve was controlled, in part, by vacuum drawn from the [[Carburetor|carburetor's]] venturi, which allowed more precise constraint of EGR flow to only those engine load conditions under which {{NOx}} is likely to form.<ref name=MTSC_73-1>[http://www.imperialclub.com/Repair/Lit/Master/302/page07.htm "1973 Cleaner Air System Highlights" – Chrysler Corporation] {{Webarchive|url=https://web.archive.org/web/20150924034014/http://www.imperialclub.com/Repair/Lit/Master/302/page07.htm |date=24 September 2015 }}, imperialclub.com</ref> Later, [[backpressure]] [[transducer]]s were added to the EGR valve control to further tailor EGR flow to engine load conditions. Most modern engines now need exhaust gas recirculation to meet {{NOx}} emissions standards. However, recent innovations have led to the development of engines that do not require them. The 3.6 [[Chrysler Pentastar engine]] is one example that does not require EGR.<ref>{{Cite web |url=http://www.autoguide.com/auto-news/2010/10/2011-dodge-challenger-officially-revealed-with-305-hp-pentastar-v6.html |title=2011 Dodge Challenger Officially Revealed With 305-HP Pentastar V6 |work=autoguide.com |date=22 October 2010 |access-date=26 September 2011 }}</ref>